This invention relates generally to wind turbines, and more particularly to methods and apparatus for efficiently reducing rotor loads in a wind turbine upon detection of a blade-pitch failure and loss of counter-torque, for example, a grid anomaly.
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 80 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid. Some turbines utilize generators that are directly coupled to the rotor without using a gearbox.
A backup power source, such as a battery, is provided in the rotor blade adjustment system and is directly applied to the blade adjustment drive when the power grid fails, thereby assuring power at all times to the blade adjusting drive. If an emergency shut down is triggered, which usually requires auxiliary-powered blade pitching, all blades start pitching towards feather with the same fast pitch rate.
At least some known wind turbines may include redundant braking systems to satisfy safety requirements. For example, at least some known wind turbines include a disk brake to facilitate stopping a wind turbine rotor against full wind torque, and a stored energy source, such as hydraulic accumulators, stored spring energy, capacitors, and/or batteries, to enable braking during a power failure.
One of the most severe situations for a wind turbine is a combination of the loss of counter-torque provided by the generator, for example, loss of grid connection, and the failure of one of the blades to pitch towards feather. The pitch failure causes blade asymmetry and the loss of counter-torque causes high rotor speed. The combination of these two conditions causes a large aerodynamic imbalance in the rotor that may cause very high loads in many components in the wind turbine, particularly in the tower top (e.g., hub, main bearing, main frame, and the like). Thus, there is a need to minimize the effect of these very high loads on wind turbine components during detection of blade-pitch failure in association with the loss of counter-torque.